What is a tandem axle?
A tandem axle is two axles spaced close together (typically 40–96 inches) that share a load as one group. The US federal tandem limit is 34,000 lb.
A tandem axle is a pair of axles mounted close together — usually 40 to 96 inches apart — that carry a load as a single group. The drive axles on a tractor and the rear axles on a semitrailer are almost always tandems.
The limit
Under US federal Interstate rules a tandem group is capped at 34,000 lb. Because the two axles are close together, the Bridge Formula may allow slightly less depending on the exact spacing. When axles in a group sit between about 40 and 96 inches apart they count as a tandem for weight purposes.
Adding a third axle to the group makes it a tridem, which raises the group's allowance. Sliding a tandem forward or back is the usual way to move weight between axle groups and bring an overloaded axle back within limits.
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Related
What is a tridem axle?A tridem axle is a group of three axles spaced close together. Common in Canada, where a tridem group can be rated to roughly 21,000–24,000 kg by its spread.What is axle weight?Axle weight is the portion of a vehicle's total weight carried by a single axle or axle group — the number weigh stations and the Bridge Formula regulate.What is the Federal Bridge Formula?The Federal Bridge Formula (W = 500(LN/(N-1)+12N+36)) sets the maximum legal weight on any group of axles based on how far apart they are spread.Axle weight limit checker (federal)Check steer, drive, and trailer axle weights plus gross weight against the US federal limits: 20,000 lb single, 34,000 lb tandem, 80,000 lb gross.